Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that at least two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized and have led various government authorities to restrict industrial emissions in an attempt to limit the level of the oxides in the atmosphere. Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation. EQU 2NO+O.sub.2 .revreaction.2NO.sub.2
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, other nitrogen oxides, and mixtures thereof.
Formation of man-made nitrogen oxides from the elements occurs in the high temperature zones of combustion processes. The internal combustion engine, and coal or gas-fired or oil-fired furnaces, boilers and incinerators, all contribute to NO.sub.x emissions. In general, fuel-rich combustion mixtures produce exhaust gases with lower contents of NO.sub.x than do lean mixtures. Although the concentrations of NO.sub.x in the exhaust gases produced by combustion usually are low, the aggregate amount discharged in industrial and/or highly populated areas is adequate to cause problems. Other industrial sources of pollution also exist. These are associated with the manufacture of nitric acid, with nitration of organic chemicals, and with other chemical operations such as the reprocessing of spent nuclear fuel rods by dissolution in nitric acid to recover uranyl nitrate followed by calcination to convert the nitrate to uranium oxide. In these instances the waste gases may contain relatively high levels of NO.sub.x, approaching 3%.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. It has been discovered, however, that adding a reductant such as ammonia to the exhaust gas can, under appropriate reaction conditions, convert NO.sub.x to elemental nitrogen and steam.
The process of adding ammonia to industrial flue gas followed by contact with a catalyst at a temperature in the range of about 200.degree.-600.degree. C. to denitrify the flue gas has come to be known as the process for Selective Catalytic Reduction (SCR) of NO.sub.x. In order to avoid confusion, any reference made herein to "Selective Catalytic Reduction", or to "SCR", is intended to refer to only that process in which a mixture of NO.sub.x and NH.sub.3 are induced to react catalytically at elevated temperatures, and to exclude processes in which other reductants such as CO or hydrogen gas are substituted for NH.sub.3.
The term "denitrify" as used herein, means to reduce the amount of one or more noxious nitrogen compounds (such as NO, NO.sub.x and HCN) contained in a waste gas, preferably by conversion to nitrogen gas.
The use of zeolite-based catalysts for the SCR of nitrogen oxides with ammonia is well established. For example, U.S. Pat. No. 4,220,632 to Pence et al. discloses a process for reducing noxious nitrogen oxides from a fossil-fuel-fired power generation plant, or from other industrial plant off-gas streams, to elemental nitrogen and/or innocuous nitrogen oxides employing ammonia as the reductant and, as the catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms.
U.S. Pat. No. 5,173,278 to Marler et al. discloses an SCR process where the ammonia needed for the reduction of NO.sub.x is generated, at least in part, by hydrolysis of HCN over a supported transition metal and/or a crystalline zeolite catalyst. The process described in this patent appears to require that HCN be present.
In particular, it is known that the hydrogen form of ZSM-5 (HZSM-5) is well suited for this reaction at temperatures between about 400.degree.-500.degree. C. U.S. Pat. No. 4,778,665 to Krishnamurthy et al. describes an SCR process for pretreating industrial exhaust gases contaminated with NO.sub.x in which the catalyst has a silica to alumina ratio of at least about 20 and a Constraint Index of 1 to 12. The entire contents of this patent are incorporated herein by reference as if fully set forth.
At temperatures below about 400.degree. C., HZSM-5 is significantly less efficient at removing nitrogen oxides from the gas stream.